Orange County, California (CNN) - "Hello? Hey, what are you doing, girl? You just woke up? You going to be free to hang out in a little bit?" Shane, a vice unit undercover investigator, is on the phone with a woman who placed an online ad offering adult services.

"Okay I'm going to head down to the Disneyland area and get a hotel." He's making a date, and choosing his words carefully.

"I just want to make sure I get what I need. Are you bringing condoms or do I need to bring condoms? You've got some? And it's 200 for an hour right?" Shane has become an expert at scoring that important criminal admission over the phone - making sure there is no confusion that sex is expected on this date.

"From what I found, sometimes you can use too much jargon," Shane explained. "If you use too many street terms you can come off like a cop so I almost talk to them like, "Hey this is what I'm looking for" - just common terms and maybe throw in just a little bit of street jargon.

"If you call them rude or real vulgar they'll just hang up on you. So, to them it's a business and they run it like it's a business, so there's that fine dance you have to do with them in negotiation you have to play to get the deal to work."

In a series this week, CNN shows the struggle against human trafficking through the eyes of the investigators at Mossos d'Esquadra, the police agency for the Catalonia region of Spain.

They wear football jerseys, T-shirts, jeans and sneakers. They look like ordinary customers having a beer at the corner pub but that ability to blend in is also key to their role in the fight against human trafficking.

They are the men and women of an elite human trafficking unit in Spain's Catalonia region and they have to get key players in criminal gangs to trust them.

The region is a hot spot for traffickers. Barcelona - its biggest city and one of Europe's marquee tourist destinations - provides a cloak for traffickers who bring victims in on tourist visas.

Large-scale criminal organizations from Eastern Europe, Africa and China are setting up shop - bringing people into Spain under the guise of giving them jobs, then keeping their passports and forcing them to work in nightmarish conditions, either in prostitution or labor exploitation. (Read more about Spain's hot spot for human trafficking)

It has kept the Mossos d'Esquadra undercover unit, which is formally called the Central Unit Against Trafficking of Human Beings, very busy. FULL POST

In a series this week, CNN shows the struggle against human trafficking through the eyes of the investigators at Mossos d'Esquadra, the police agency for the Catalonia region of Spain.

Barcelona is one of the most popular tourist destinations in southern Europe and the largest city in Spain's Catalonia region.

The location's beauty and mild weather draw tourists, but traffickers like the region, too: Vacationers can become clients for prostitution and traffickers also use tourist visas to bring people into the country.

About the Freedom Project

CNN is joining the fight to end modern-day slavery by shining a spotlight on the horrors of modern-day slavery, amplifying the voices of the victims, highlighting success stories and helping unravel the complicated tangle of criminal enterprises trading in human life. WHY WE'RE DOING THIS | MORE ABOUT THE PROJECT

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